H Grand Empress lockout problems

My 10 man guild killed the first 4 HoF bosses on Heroic two weeks ago. Last week, we extended the lockout to continue working on H Amber Shaper. We didn't kill H Amber Shaper last week, so we extended the lockout again for this week.

We killed H Amber Shaper tonight, but we couldn't engage H Empress. We cleared the trash and she wouldn't spawn. I was under the impression that if you continuously extended the same lockout, you should still be able to engage the last boss on Heroic if you killed all the previous bosses on Heroic too (which we did). I'm extremely confused about the HoF lockout problem because we extended for H Spirit Kings for two weeks and after we killed him and H Elegon, we were able to engage H Will of the Emperor.